See how instructional the Mudcat Cafe can be?

Actually you CAN own a tank and people do, in both the US and the UK. You might even be able to leave the cannon serviceable (in the US). Possessing the ammunition, since it would include explosives, would be another story. The machine gun(s) would have to be rendered unusable, generally by welding the breech and/or plugging the barrel and welding it in place. Cannons on display, whether on tanks or otherwise, almost always have the breeches welded shut for liability reasons -- imagine the lawsuit is a child playing on a howitzer had the breech closed on their hand!

On the other hand, you CAN legally possess a cannon in the US and I know some (including my own brother) who do. These would be muzzle-loaders, such as might have been used in the US Civil War (or the British one, for that matter). There are regular cannon shooting competitions held by some of the Civil War groups, etc.